The present invention relates, in general, to controlling high voltage transistors, and more particularly, to a low voltage circuit for controlling a high voltage transistor that operates a high voltage motor.
Transistor circuits had long been used by the industry for operating electric motors. Of particular interest in recent years, was the use of transistor circuits to operate high voltage, generally in excess of 200 volts, motors. The recent interest had been in not only operating these motors at a constant speed, but also varying the speed of the motor. Generally a pulse width modulation (PWM) technique was used to generate timing signals which were applied to high frequency transistors that operated the motor. This technique of controlling high voltage motors required electronics which had to operate at the high voltages of the motor and had to withstand voltage and current transients resulting from the operation of the motor. Previous methods to control these motors by using PWM signals generally divided the electronics into two electrically isolated sections. One section operated at low voltages(5 volts to 20 volts) and a separate section operated from a voltage approximately equal to the motor voltage. The low voltage section generally contained electronics for developing the PWM timing signals and the high voltage section generally controlled the motor. Since it was difficult and expensive to produce electronics capable of operating at the high voltages of the motor, the electronics in the high voltage section was kept to a minimum. Signals between the two sections were then connected by a device that maintained isolation between the sections such as a transformer or optical coupler. Both optical couplers and transformers limited the switching speed of the PWM signals that were connected between the two sections of electronics. The isolation components alone added additional costs to the circuit, while additional costs were added from isolating the two sections with floating or isolated power supplies. The previous circuits were also difficult to fabricate in an integrated circuit which further increased the cost of the implementations.
Accordingly, it would be desirable to have a low voltage circuit for operating a high voltage motor wherein the circuit can be formed into an a integrated circuit, and the circuit eliminates the need for both separate isolated power supplies and the need for isolation components such as optical couplers and transformers, thereby improving the switching time and lowering the cost of the circuit.